Optical Switching Architectures for Nodes in WDM Mesh and Ring Networks

ABSTRACT

Switching architectures for WDM mesh and ring network nodes are presented. In mesh networks, the switching architectures have multiple levels—a network level having wavelength routers for add, drop and pass-through functions, an intermediate level having device units which handle add and drop signals, and a local level having port units for receiving signals dropped from the network and transmitting signals to be added to the network. The intermediate level device units are selected and arranged for performance and cost considerations. The multilevel architecture also permits the design of reconfigurable optical add/drop multiplexers for ring network nodes, the easy expansion of ring networks into mesh networks, and the accommodation of protection mechanisms in ring networks.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/208,870, filed Aug. 12, 2011, which in turn is a divisional of U.S.patent application Ser. No. 11/278,595 filed Apr. 4, 2006, entitled“Optical Switching Architectures for Nodes in WDM Mesh and RingNetworks.” The entirety of each of these applications is incorporatedherein by reference.

BACKGROUND

The present invention is related to WDM (Wavelength DivisionMultiplexing) optical systems and, more particularly, to switchingarchitectures of nodes for handling optical channels in WDM networksystems.

WDM refers to network systems in which multiple optical signals havingdifferent wavelengths can share an optical fiber, each wavelengthdefining a particular communication channel. In a stricter sense, WDMalso refers to an ITU (International Telecommunications Union) standardwhich includes the specification of the particular channel wavelengthsand the spacings between these channels. DWDM (Dense WDM) refers to amore recent ITU standard in which the channel spacings are tighter sothat more wavelength channels can be packed into an optical fiber. Itshould be noted that the term WDM, as used herein, refers to the first,more inclusive sense so as to include the ITU WDM and DWDM standards,unless specifically stated otherwise.

WDM has many advantages for optical communication systems includingincreased capacity. A representative WDM network may include many nodesconnected to one another by optical fibers in a mesh or in a ringarrangement. At each node typically, only a portion of the wavelengths(also referred to as WDM channels) are used for transmission andreception, while the other wavelengths remain untouched as“pass-through” channels. For reception, a node isolates and removes (or“drops”) these particular channel signals from the light flow in anoptical fiber for processing by receiver circuitry within the node orfor otherwise rerouting the signals; for transmission, the nodegenerates or routes (or “adds”) particular channel signals generatedelsewhere into the light flow in an optical fiber for transmission todesignated destinations over the network. Besides these add and dropfunctions, many nodes have switching functions by which signals in onechannel carried in one optical fiber are switched to a different fiber,by which signals in one wavelength channel are switched to a differentwavelength channel, or by which signals in a wavelength channel areswitched to a different optical fiber in a different wavelength channel.

Heretofore, switching architectures for such nodes have been directedtoward achieving full functionality with resulting high costs. This hasimpeded the adoption of optical networks and the advantages of largebandwidth in telecommunication networks. Alternatively, some switchingarchitectures with low costs have been advocated, but with limitedfunctions and utility.

Based upon newly emerging technologies, the present invention providesfor switching architectures for nodes in which different functionallevels are separated by interfaces. This allows each level to beconstructed and treated as a module. This construction allows noderepairs to be made easily. Furthermore, the node can be upgraded easilyand systematically with sufficient functionalities to provide thedesired utility to the user, i.e., the node has the desired functions onan “as-needed” basis. Costs are contained to encourage the adoption ofoptical networks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a general switching architecture for nodes in a meshoptical network, according to one embodiment of the present invention;

FIG. 1B illustrates a general switching architecture for nodes in a ringnetwork;

FIG. 2 illustrates a switching architecture for nodes in a mesh opticalnetwork, according to one embodiment of the present invention;

FIG. 3A illustrates the first level of the node switching architectureof FIG. 2 in greater detail;

FIG. 3B illustrates an embodiment of the second, or intermediate, levelof the node switching architecture of FIG. 2 in greater detail;

FIG. 3C illustrates the third level of the node switching architectureof FIG. 2 in greater detail;

FIG. 4 illustrates multiplexers and demultiplexers used as componentsfor the intermediate level of the node switching architecture, accordingto another embodiment of the present invention;

FIG. 5 illustrates wavelength routers used as components for theintermediate level of the node switching architecture, according toanother embodiment of the present invention

FIG. 6 illustrates couplers and wavelength routers used as componentsfor the intermediate level of the node switching architecture, accordingto another embodiment of the present invention;

FIG. 7 illustrates an intermediate level with a multiplexer and ademultiplexer as shown in FIG. 4, plus a switched multiplexer and aswitched demultiplexer for additional add/drop functionality, accordingto still another embodiment of the present invention;

FIG. 8 shows an embodiment of the present invention for ring networknodes at which signals can be added and dropped;

FIG. 9 shows another embodiment of the present invention for ringnetwork nodes to be expanded; and

FIG. 10 shows a switching architecture for ring network nodes whichpermits easy implementation of protection mechanisms, according toanother embodiment of the present invention.

DETAILED DESCRIPTION

FIGS. 1A and 1B illustrate nodes in mesh and ring networks respectively.In a mesh, a single representative node 10 in FIG. 1A is connected byoptical fibers which carry optical signals to and from the node in aplurality of directions to other nodes of the network. The double-headedarrows 11-18 represent at least two optical fibers, one optical fiber tocarry signals in each direction. The arrow 11 illustrates opticalsignals to, and from, the node 10 from, and to, the “west” direction.The arrow 12 illustrates optical signals to, and from, the node 10 from,and to, the “east” direction. Signals to and from the “north” directionsare indicated by an arrow 13 and signals to and from the “south”directions are indicated by an arrow 14. The arrows 15-18 indicatesignals in the “northeast,” “southwest,” “northwest,” and “southeast”directions respectively, and also are representative of a generalizedmesh in which signal directions are not limited to north/south,east/west directions.

On the other hand, a ring network can be considered a degenerate case ofa mesh network in which the plurality of signal directions to and from anode is reduced to only two directions. Representative nodes 20 in FIG.1B are connected by optical fibers which carry optical signals in “west”and “east” directions to other nodes (not shown) in a ring, i.e., aclosed loop 19. Arrows show that ring 19 has at least two opticalfibers, each optical fiber carrying signals in opposite directionsaround the ring.

According to the present invention, the devices in these mesh and ringnodes are organized into levels with interfaces between the levels. Eachlevel is modularized so that the repair of a node can be made easily.Furthermore, to meet the desired functional capabilities a node can beeasily and economically up-graded (or down-graded) by replacing selectedmodules, rather than replacing the entire node.

In the architecture of the present invention, a node is divided into atleast two levels or, more likely, three levels. In a three-levelarchitecture, the node is divided into a first (or network) node, asecond (or intermediate) node, and a third (or local) level. In atwo-level architecture, the devices and functions of the intermediatelevel are split between the network and local levels. The differentlevels of the node architecture of the present invention are explainedand described below in detail: first, with respect to three levelarchitectures in mesh networks; secondly, with respect to two-levelarchitectures in mesh networks; thirdly, with respect to three-levelarchitectures in ring networks; and finally, with respect to two-levelarchitectures in ring networks.

Organization of Three-Level Architectures for Mesh Network Nodes

A node switching architecture for a mesh network is shown in FIG. 2. Inaccordance with the present invention, the architecture is organized inthree levels as separated by the two vertical dotted lines.

At a first, or network, level, wavelength routers 21T-24T and 21F-24Fselectively direct the movement of WDM optical signals through thenetwork node, i.e., the node “pass-through” signals. The wavelengthrouters 21T-24T and 21F-24F also selectively add local signals to, andselectively drop network signals from, the network. The signals to beadded and dropped are received from, and sent to, device units 31T-34Tand 31F-34F in a second, or intermediate, level of the switchingarchitecture. The device units 31T-34T and 31F-34F in turn receive thesignals to be added from, or send the signals to be dropped to, portunits 30 a-30 i in a third, or local, level of the switchingarchitecture.

Returning to the network level of the switching architecture, theoptical fibers of the network are shown as running in only four“directions,” north, south, east and west for simplicity's sake. Itshould be understood that more directions in the mesh are possible. Thewavelength routers 21T and 21F handle signals to and from the westdirection. The wavelength router 21T sends signals to (T) the west; thewavelength router 21F receives signals from (F) the west. Likewise, thewavelength routers 22T and 22F handle signals to and from the eastdirection; the wavelength routers 23T and 23F handle signals to and fromthe north direction; and the wavelength routers 24T and 24F handlesignals to and from the south direction.

The wavelength router 21T selectively adds local signals to the westdirection; the wavelength router 22T selectively adds local signals tothe east direction; the wavelength router 23T selectively adds localsignals to the north direction; and the wavelength router 24Tselectively adds local signals to the south direction. On the otherhand, the wavelength router 21F selectively drops signals from the westdirection; the wavelength router 22F selectively drops signals from theeast direction; the wavelength router 23F selectively drops signals fromthe north direction; and the wavelength router 24F selectively dropssignals from the south direction.

The signals to be added are sent from the device units 31T-34T and thedropped signals are received by the device units 31F-34F in the second,or intermediate, level of the switching architecture node. As describedin further detail below, the device units 31T-34T and 31F-34F arecreated from different device components and arranged in differentcombinations of device components to meet the desired performance andcosts target for the node.

The port units 30 a-30 i form the third, or local, level of theswitching architecture for the signals dropped from the network or thelocal signals to be added to the network transport. Since the number ofport units depends upon the particular devices or arrangement of deviceswhich form the device units of the intermediate layer, FIG. 2 shows anindeterminate number of port units 30 a-30 i. Each port unit 30 a-30 iincludes a transmitter which converts local electrical signals intooptical signals for network transmission and receivers which convertoptical signals dropped from the network into electrical signals.

FIGS. 3A-3C illustrate the details of the three-levels of the switcharchitecture of FIG. 2, according to one embodiment of the presentinvention. FIG. 3A shows the first network level of the node switcharchitecture. Each network direction is handled by two wavelengthrouters, a “T,” or “to,” wavelength router, which directs WDM opticalsignals to the particular direction, and an “F,” or “from,” wavelengthrouter, which accepts signals from the particular direction. The Twavelength router of each direction is connected to the outputs of the Fwavelength routers of the other three directions. For example, thewavelength router 23T, which sends signals to the north direction fromthe node, receives signals from the west wavelength router 21F, the eastwavelength router 22F and the south wavelength router 24F. The Fwavelength router of each direction is connected to inputs of the Twavelength routers of the other three directions. For example, thewavelength router 22F, which receives signals from the east direction,sends signals to the west wavelength router 21T, the north wavelengthrouter 23T, and the south wavelength router 24T.

For the add and drop functions, the T wavelength router to eachdirection also receives Add input signals from the intermediate layerand the F wavelength router from each direction also sends Drop signalsto the intermediate layer. In the intermediate stage, the device units31T-34 T manage the signals which are to be added to the network in thewest, east, north and south directions respectively. The device units31F-34F manage the WDM signals which are to dropped from the networkfrom the west, east, north and south directions respectively.

The details of one embodiment of the device units 31T and 31F for westdirection signals are shown in FIG. 3B and are representative of thedevice units 32T-34T and 32F-34F for the other network directions. Thedevice unit 31T is formed from a coupler 36 and an amplifier 35. Thecoupler 36 is connected to, and receives signals from, the third, orlocal, level and combines these signals into a single fiber which is aninput to the amplifier 35. The coupler 36 has input terminals to receivesignals for all the wavelength channels which may be carried on anetwork optical fiber. Currently WDM networks carry 32 wavelengthchannels. After being amplified by the amplifier 35, the combinedsignals are sent to the west direction wavelength router 21T of thefirst level.

The device unit 31F is formed by a demultiplexer 37 and a switch 38. Thedemultiplexer 37 is connected to, and receives signals from, the firstlevel west wavelength router 21F. The demultiplexer 37 separates thesignals which are dropped by the wavelength router 21F into wavelengthchannel signals which are passed to input terminals of the switch 38.The switch 38 has a 32×32 switching capacity with the assumption that anoptical fiber can carry up to 32 wavelength channels so that the switch38 can selectively place any wavelength channel at any of the switch'soutput terminals (and ports).

A port unit 30 a, which is representative of all the ports 30 a-30 i ofthe third, or local, level is shown in FIG. 3C. (Note that with the32×32 switch 38, the total number of port units 30 a-30 i can equal 32.)For sending signals to the intermediate level, the port unit 30 a has atransmitter 41 with a tunable laser 43 and a 1×N switch 45T. Thetransmitter 41 receives electrical signals and converts them to opticalsignals by the laser 43 which is set to a particular wavelength channelfor the port unit 30 a. The output of the transmitter 41 is sent to theswitch 45T which in turn is set to pass the transmitter signals to oneof its N output terminals. N is typically equal to the number ofoutgoing directions from the network node, four, in this example so thatthe switch 45T is connected to the intermediate level device units31T-34T for the east, west, north and south directions. The transmittedsignals from the port unit 30 a can be placed in any wavelength channelby the tunable laser 43 and sent in any direction by the switch 45T.

For signals received from the intermediate level, the port unit 30 a hasan N×1 switch 45F and a receiver 42. The switch 45F is connected to theintermediate level device units 31F-34F for the east, west, north andsouth directions. With each of the device units 31F-34F having a 32×32switch 38 as shown in FIG. 3B, the port receiver 42 can selectivelyreceive network signals dropped from anyone of the network directions,or more precisely, the port receiver of anyone of the ports unit 30 a-30i can receive network signals dropped from anyone of the networkdirections.

Thus the node switch architecture illustrated in FIGS. 3A-3C is highlyflexible. By setting the tunable lasers 43 in the local port units 30a-30 i, the switches 38 in the intermediate device units 31F-34F and thewavelength routers 21F-24F of the first level, any local port unit 30a-30 i can send signals on any wavelength channel in any networkdirection. Likewise, by setting the switches 38 in the device units31F-34F and the switch 45F of a port unit 30 a-30 i, any local port unitcan receive signals on any wavelength channel from any networkdirection. Control of the tunable laser 43, switches 45T and 45F andswitch 38 can be set manually or by computer programming by a controlunit (not shown) with control lines to these elements.

Most of the device components described above are readily available andwell-known to optical network designers and engineers. The wavelengthrouters used in the network level are devices which can direct signalsreceived at any input port to any output port by wavelength. Thus thewavelength routers 21T-24T and 21F-24F can manage the pass-through andadd/drop functions for the node. Furthermore, optical performance isacceptable with pass through internal losses (which include lossesthrough two wavelength routers) around 8 dB. Wavelength routers areavailable from companies, such as Capella Photonics of San Jose, Calif.,and Metconnex, Inc. of Ottawa, Ontario, Canada.

Since wavelength routers have a limited number of ports, typicallyaround 10, the costs of such devices are relatively low in comparison totheir switching responsibilities. On the other hand, switches with alarge number of ports, such as the 32×32 switch 38 of FIG. 3B, arerelatively expensive compared to the other components of the describedarchitecture. MEMS (Micro-Electro-Mechanical Systems) technology istypically employed in such switches which allow for maximumfunctionality so that signals at any one of the numerous switch inputports can be sent to anyone of the numerous output ports.

The present invention also offers less expensive alternatives to thepreviously described node switching architecture while maintaining muchof its flexibility. In FIG. 4 a demultiplexer 51 is used for therepresentative device unit 31F and a multiplexer 52 for therepresentative device unit 31T in the intermediate level, according toanother embodiment of the present invention. Each of the input ports ofthe multiplexer 52 is connected to the transmitter 41 and switch 45T ofone of the local port units 30 a-30 i and the multiplexer's output portis connected to its corresponding first level wavelength router 21F.Likewise, each of the output ports of the demultiplexer 51 is connectedto the switch 45F and receiver 42 of the corresponding local port unit30 a-30 i, and the demultiplexer's input port is connected to itscorresponding first level wavelength router 21T. Of course, thiscombination of components for the intermediate level device units31T-34T and 31F-34F is not as flexible as the combination of componentsillustrated in FIG. 3B. Unlike the output ports of the switch 38, thewavelength channels of the output terminals of the demultiplexer 51cannot be changed. Furthermore, the number of output terminals islimited compared to the full number of channels on a fiber since thesignal strength on each output terminal of a demultiplexer iscorrespondingly reduced by the number of output terminals. The number ofport units 30 a-30 i must be reduced accordingly. Nonetheless, if theadd/drop requirements for the node are not high, these intermediatelevel device units 31T-34T and 31F-34F provide satisfactory performanceat a much lower cost than the device units represented in FIG. 3B.

In place of the intermediate level demultiplexer and multiplexer of FIG.4, wavelength routers are used in FIG. 5, according to anotherembodiment of the present invention. A wavelength router 53 is used forthe device unit 31F and a wavelength router 54 for the device unit 31T.Each of the input ports of the wavelength router 54 is connected to thetransmitter 41 and switch 45T of one of the local port units 30 a-30 iand the wavelength router's output port is connected to an input port ofits corresponding first level wavelength router 21F. Likewise, each ofthe output ports of the wavelength router 53 is connected to the switch45F and receiver 42 of the corresponding local port 30 a-30 i and thewavelength router's input port is connected to an input port of itscorresponding first level wavelength router 21T. While theseintermediate level components are flexible so that the wavelength router53 can selectively switch the wavelength channel of its output port andconnected local port units 30 a-30 i, the number of output (or input)ports of a wavelength router is fairly limited. This sharply restrictsthe number of local port units 30 a-30 i. To increase the number oflocal port units, additional wavelength routers can be added on anyavailable (unused) output ports of the first level wavelength router 21Fand the input ports of the wavelength router 21T. This allows the numberof port units for add/drop functions to be increased in a modularfashion, on an “as-needed” basis, so that costs are incurred when thesystem is expanded. Nonetheless, the wavelength routers of FIG. 5 aremore expensive than the multiplexers and demultiplexers of FIG. 4.

The addition of wavelength routers for expanding the number of portunits 30 a-30 i can be seen in another embodiment of the presentinvention in FIG. 6. Combinations of couplers and wavelength routers areused for the intermediate level. A coupler 55 and one or more wavelengthrouters 57 are used for the device unit 31F, and a coupler 56 and one ormore wavelength routers 58 for the device unit 31T. Each wavelengthrouter 57 has an input port connected to an output port of the coupler55 and its output ports connected to switch 45F and receiver 42 of aport unit 30 a-30 i. Each wavelength router 58 has an output portconnected to an input port of the coupler 56 and its input portsconnected to switch 45T and transmitter 41 of the corresponding port 30a-30 i. As more ports units are needed, more wavelength router 57 and 58are respectively connected to the output ports of the coupler 55 and theinput ports of the coupler 56. Furthermore, the switches 45T for theport units 30 a-30 i can be replaced by cheaper couplers since thewavelength router(s) 58 can perform the functions of the switches 45T.While less expensive to implement than the FIG. 5 arrangement, theadd/drop insertion losses of the FIG. 6 embodiment are greater due tothe couplers 55 and 56 (and couplers substituting for the switches 45Tin the port units 30 a-30 i). Hence optical performance for the add/dropfunctions is sacrificed for lower costs.

Another embodiment of the present invention is illustrated in FIG. 7which also permits the optional increase in the number of local add/dropport units 30 a-30 i. Like the arrangement in FIG. 4, a multiplexer 61and demultiplexer 62 are arranged in the intermediate layer to connectlocal port units 30 a-30 i in the third layer. If more local port unitsare needed, the wavelength routers 21F-24F and 21T-24T in the firstlayer drop and add more wavelength channels through their unused outputand input ports. In the intermediate layer a switched multiplexer 64 isconnected to the newly operational input ports of the wavelength routers21T-24T and a switched demultiplexer 63 is connected to the newlyoperational output ports of the wavelength routers 21F-24F. A switcheddemultiplexer is the demultiplexer/switch combination shown in FIG. 3B.A switched multiplexer is a combination of a switch with its outputterminals connected to the input terminals of an optical multiplexer. Ofcourse, the costs per channel through the switched multiplexer 64 anddemultiplexer 63 are higher than those through multiplexer 61 anddemultiplexer 62 and their fixed local port units. The switchedmultiplexer 64 and demultiplexer 63 can be used for higher valueservices, such as channels requiring optical protection.

Switching Architectures for Nodes in Ring Networks

The multi-level switching architecture of the present invention isfurther adaptable to ring networks, the degenerate case of amulti-directional mesh of optical fibers reduced to optical fiberscarrying signals in two opposite directions. As a matter of terminology,it should be noted that combinations of device components which providefor the add, drop and pass-through functions of a node of a ring networkare often called optical add/drop multiplexers (OADMs). OADMs whichallow the added, dropped or passed-through wavelength channels to bechanged are termed reconfigurable add/drop multiplexers (ROADMs). Thepresent invention provides for tri-level architectures for ROADMs whichare flexible in view of their costs, capable of expanding a ring networkinto mesh network, and suitable for network protection mechanisms.

A switching architecture for an ROADM, according to an embodiment of thepresent invention, is partially illustrated in FIG. 8 which shows onlyone ring optical fiber 70. Most ring networks, such as those runningunder SONET/SDH protocols, have at least a second optical fiber in whichsignals travel in the opposite direction from the first fiber. The FIG.8 architecture includes a coupler 71, a demultiplexer 73 and a switch 75for the drop function of the node, and a wavelength router 72, a coupler74 and an amplifier 76 for the add function of the node. The coupler 71and wavelength router 72 operate at the first level of architecture tomanage the add, drop and pass-through operations for the optical signalson the ring network.

For the drop function, the coupler 71 splits the signals in the fiberand directs one set of signals to the wavelength router 72 and a secondset of signals to the demultiplexer 73 and switch 75, which operate atthe intermediate level of the architecture. The demultiplexer 73separates the signals by their wavelengths and the 32×32 switch 75selectively directs the wavelength signals to its output ports which inturn are connected to port units 30 a-30 i (shown in FIG. 3C) operatingat the third, or local, level of the architecture. In the presentembodiment, the switches 45T and 45F of the port units (see FIG. 3C) are1×2 and 2×1 respectively and each switch 45F which is connected to thereceiver 42 of the port unit has one of its two input ports connected toone of the output ports of the switch 75. The second input port of theswitch 45F is connected to an output terminal of a corresponding switch75 for the second network optical fiber (not shown) for carrying signalsin the opposite direction.

The wavelength router 72 selects the wavelength signals from the coupler71 which are to be passed-though the node. For the add function, thewavelength router 72 also selectively adds the amplified optical signalsfrom the coupler 74 and the amplifier 76, which operate at theintermediate level of the architecture, to the optical fiber 70. Each ofthe input ports of the coupler 74 is connected to an output port of theswitch 45T of the port elements 30 a-30 i which is also connected to theswitch 75. The second output port of the switch 45T is connected to aninput port of a coupler 74 for the second optical fiber (not shown).

The resulting ROADM has an expensive component, i.e., the switch 75.Nonetheless, signals through the switch 75 and the wavelength router 72are controllable so that locations of wavelength channels can be movedabout the add/drop port units 30 a-30 i and different wavelengthchannels selected for passing through the node.

The switching architecture of the present invention also provides forthe ability to expand a ring network into a mesh network. In FIG. 9 thesame optical fiber 70 carrying signals from the west-to-east directionas in FIG. 8 is used. In this example, however, representative opticalamplifiers 78 and 79, such as erbium-doped fiber amplifiers, typicallyused to maintain signal strength in WDM network fibers are also shownwith a representative OADM 80. The OADM 80 is not necessarily the samearchitecture as the one shown in FIG. 8, and, in fact, is different forpurposes of illustrating this aspect of the present invention.

In accordance with this embodiment, simple couplers 81 and 82 areconnected to the optical fiber 70 for optional network expansion. Thecoupler 81 can split the signals on the optical fiber 70 and the coupler82 can add signals to the optical fiber 70. When a wavelength router 83is connected to the coupler 81, a set of signals from the optical fiber70 can be selectively sent to optical fibers in new network directionsto form a mesh. Likewise, a wavelength router 84 can be connected to thecoupler 82 so that signals from the new network directions can be addedto the optical fiber 70. The couplers 81 and 82, and wavelength routers83 and 84 become part of the first level of the switching architectureof a node in a network mesh. There is no need to change the componentsof the OADM 80 for the west-to-east optical fiber 70 and thatmultiplexers and demultiplexers can be added for add/drop functions inthe new network directions. Also, though only one optical fiber 70 isshown for the ring network, it is readily understood that the describedembodiment also applies to other network optical fibers not shown forthe ring network.

FIG. 10 illustrates the adaptability of the present invention to theprotection mechanisms, such as 1+1 and 1:1, found in optical networks.In this example, a heavily lined ring 89 indicates a plurality ofoptical fibers which typically form a ring network. The ring 89 hasnodes with ROADMs (Reconfigurable Optical Add/Drop Multiplexers) 85 bywhich selected optical signals are added and/or dropped from the ring89. With a node switching architecture according to the presentinvention, the network protection mechanisms are extended to the localport(s) to which the network signals are dropped or from which localsignals are added.

The particular architecture of the ROADM 85 is not pertinent to thisaspect of the present invention, but any ROADM has interfaces for eachoptical fiber in the ring 89: An example of an ROADM interface for oneoptical fiber are the input terminals to the wavelength-selective switch77 and the output terminals of the demultiplexer 73 of the ROADM 80illustrated in FIG. 9. With two optical fibers of the ring 89 in FIG. 10termed as running in the east and west directions, the ROADM 85 has twointerfaces 85A and 85B, one for each optical fiber. The ROADM 85 and itsinterfaces 85A and 85B are considered the first level of the nodeswitching architecture as delineated by an upper dotted line runninghorizontally. Each of the interfaces 85A and 85B is connected to switchmatrices 86A and 86B respectively. The two matrices 86A and 86B, whichform the intermediate level of the architecture, are connected to aplurality of third level port units, one of which is shown in FIG. 10and delineated by a lower dotted line running horizontally. Theillustrated port unit has two splitter/switches 87A and 87B, and atransmitter/receiver unit 88 which has its transmitter portion connectedto the splitter/switch 87 A and its receiver portion connected to thesplitter/switch 87B.

Operationally, local signals to be transmitted over the ring 89 are sentby the transmitter/receiver 88 to the splitter/switch 87A which, as asplitter under 1+1 protection, sends the signals to both switch matrices86A and 86B. Under 1:1 protection, the splitter/switch 87A is a switchwhich selectively sends signals to either switch matrices 86A or 86B.The switch matrix 86A sends the signals to the west direction opticalfiber of the ring 89 through the interface 85A and the switch/matrix 86Bsends the signals to the east direction optical fiber of the ring 89through the interface 85B.

Under 1+1 protection, signals received from the ring 89 pass through theinterfaces 85A and 85B, the switch matrices 86A and 86B, and to thesplitter/switch 87B which is a switch selecting which signals to send tothe receiver portion of the unit 88. Under 1:1 protection, the signalsfrom the ring 89 pass through either interface 85A and switch matrix86A, or interface 85B and the switch matrix 86B, to the splitter/switch87B which is a combiner (the inverse of a splitter) to provide a pathwayfor the signals to the receiver portion of the unit 88. Hence thisarchitecture permits easy path protection mechanism.

Therefore, while the description above provides a full and completedisclosure of the preferred embodiments of the present invention,various modifications, alternate constructions, and equivalents will beobvious to those with skill in the art. Thus, the scope of the presentinvention is limited solely by the metes and bounds of the appendedclaims.

What is claimed is:
 1. A switching architecture for a node of a ringfiber optic network having optical fibers carrying wavelength divisionmultiplexed (WDM) signals to and from said node in two directions, saidswitching architecture comprising: a first network coupler for splittingWDM signals from an optical fiber of said fiber optic network; a firstnetwork wavelength router interconnected to at least one optical fibercarrying WDM signals in a direction other than said two directions, saidfirst network wavelength router for receiving said split WDM signalsfrom said first network coupler, and for passing said split WDM signalsto said at least one optical fiber carrying WDM signals in a directionother than said two directions; a second network coupler for combiningWDM signals to said optical fiber of said fiber optic network; and asecond network wavelength router interconnected to at least one opticalfiber carrying WDM signals from a direction other than said twodirections, said second network wavelength router for sending WDMsignals from at least one optical fiber carrying WDM signals from adirection other than said two directions to said second network coupler,wherein said fiber optic network can be extended beyond said twodirections.
 2. The switching architecture of claim 1, further comprisinga reconfigurable optical add/drop multiplexer connected to said opticalfiber of said fiber optic network between said first network coupler andsaid second network coupler.
 3. The switching architecture of claim 2,wherein said reconfigurable optical add/drop multiplexer furthercomprises a third network coupler for splitting WDM signals from anoptical fiber of said fiber optic network.
 4. The switching architectureof claim 3, wherein said reconfigurable optical add/drop multiplexerfurther comprises a demultiplexer coupled to said third network couplerfor demultiplexing said WDM signals split from said third networkcoupler.
 5. The switching architecture of claim 4, wherein saidreconfigurable optical add/drop multiplexer further comprises awavelength selective switch configured to selectively add wavelengths tosaid optical fiber of said fiber optic network.
 6. In a switchingarchitecture for a node of a ring fiber optic network having opticalfibers carrying wavelength division multiplexed (WDM) signals to andfrom said node in two directions, a method comprising: splitting, with afirst network coupler, the WDM signals from an optical fiber of saidfiber optic network; receiving the split WDM signals and passing thesplit WDM signals to an optical fiber carrying WDM signals in adirection other than said two directions; combining, with a secondnetwork coupler, WDM signals to said optical fiber of said fiber opticnetwork; and sending WDM signals from at least one optical fibercarrying WDM signals from a direction other than said two directions tosaid second network coupler, thereby extending said fiber optic networkbeyond said two directions.
 7. The method of claim 6, further comprisingsplitting, with a third network coupler, WDM signals from an opticalfiber of said fiber optic network.
 8. The method of claim 7, furthercomprising demultiplexing said WDM signals split from the third networkcoupler.
 9. The method of claim 8, further comprising selectively addingwavelength to said optical fiber of said fiber optic network with awavelength switch of a reconfigurable optical add/drop multiplexer. 10.A switching architecture for a node of a ring fiber optic network havingoptical fibers carrying wavelength division multiplexed (WDM) signals toand from said node in two directions, said switching architecturecomprising: a reconfigurable optical add/drop multiplexer connected tosaid fiber optic network, said reconfigurable optical add/dropmultiplexer having a first interface for WDM signals traveling in afirst direction and a second interface for signals traveling in a seconddirection, said first interface having a plurality of output ports forWDM signals dropped from said fiber optic network and a plurality ofinput ports for WDM signals to be added to said fiber optic network,said second interface having a plurality of output ports for WDM signalsdropped from said fiber optic network and a plurality of input ports forWDM signals to be added to said fiber optic network; a first switchmatrix having a plurality of interface input ports connected to saidfirst interface output ports, a plurality of interface output portsconnected to said first interface input ports, a plurality of port unitinput ports, and a plurality of port unit output ports; a second switchmatrix having a plurality of network input ports connected to saidsecond interface output ports, a plurality of network output portsconnected to said second interface input ports, a plurality of port unitinput ports, and a plurality of port unit output ports; a plurality ofport units, each port unit having a first splitter/switch having a firstoutput port connected to one of said plurality of port unit input portsof said first switch matrix, a second output port connected to one ofsaid plurality of port unit input ports of said second switch matrix,and an input port; a second splitter/switch having a first input portconnected to one of said plurality of port unit output ports of saidfirst switch matrix, a second input port connected to one of saidplurality of port unit output ports of said second switch matrix, and anoutput port; and a transmitter/receiver having a transmitter portionconnected to said input port of said first splitter/switch and areceiver portion connected to said output port of said second splitterswitch, wherein each port unit is capable of sending and receiving WDMsignals from and to said node in said two directions for protectionmechanisms on said fiber optic network.
 11. The switching architectureof claim 10, wherein said first splitter/switch comprises a splitter andsaid second splitter/switch comprises a switch so that each port unit iscapable of sending and receiving WDM signals from and to said node insaid two directions for a 1+1 protection mechanism on said fiber opticnetwork.
 12. The switching architecture of claim 11, further comprisinga transmitter/receiver unit with a transmitter portion coupled to saidsplitter and a receiver portion coupled to said switch.
 13. Theswitching architecture of claim 12, wherein said transmitter/receiverunit is configured transmit and receive local signals.
 14. The switchingarchitecture of claim 10, wherein said first splitter/switch comprises aswitch and said second splitter/switch comprises a splitter so that eachport unit is capable of sending and receiving WDM signals from and tosaid node in said two directions for a 1:1 protection mechanism on saidfiber optic network.
 15. The switching architecture of claim 14, furthercomprising a transmitter/receiver unit with a transmitter portioncoupled to said switch and a receiver portion coupled to said splitter.16. The switching architecture of claim 15, wherein saidtransmitter/receiver unit is configured to transmit and receive localsignals.